1. Technical Field
This invention relates to a hot melt type seal agent for use as an adhesive seal between a lens and a housing for an incandescent lamp such as a head lump of a vehicle.
2. Prior Art
Formerly, a lamp unit and a housing of a head lump were separate units of an assembly, whereas recently the lamp unit and the housing have become integrally one unit for the purpose of weight reduction of a vehicle and the trend to reduce the complexity of design. In the integral head lamp, a hot melt type seal agent began to be used for adhering and sealing the glass or plastic lens to the plastic or FRP housing. The hot melt type seal agent is produced by kneading a polymer, a tackifier, a softener, a filler and other miscellaneous materials. As the polymer component of the hot melt type seal agent, a simple substance or mixture of such thermoplastic resin or thermoplastic elastomer as ethylene-vinyl acetate copolymer, polyamide resin, polyethylene, ethylene-propylene rubber, styrene-isoprene-styrene (SIS) block copolymer, styrene-butadiene-styrene (SBS) block copolymer, acrylic rubber, polyisobutylene rubber, polyester elastomer and so on and have been generally used. As the softener, a simple substance or mixture of olefin with low molecular weight, atactic polypropylene, polybutene, polyisobutylene with low molecular weight, paraffinic oil, naphthenic oil and straight chain aliphatic acid ester have been used. As the tackifier, a simple substance or mixture of terpene resin, terpene-phenol resin, rosin, hydrogenated-rosin resin, various petroleum resin and so on have been used.
The hot melt type seal agent produced by kneading the above components, however, did not always give satisfactory results in adhesion, weather resistance under severe conditions and physical characteristics at high or low temperature and wet conditions though these results are necessary for the seal agent. For example, internal temperature of a lamp rises as high as 120.degree.-130.degree. C., partially rising around 140.degree. C. when the lamp is lighted. In case of a halogen sealed beam lamp, the seal agent needs a greater sealing performance as it must withstand both the pressure of the sealed gas and a higher temperature than in ordinary incandescent lamps.